1. Field of the Invention
The present invention relates to an analog-to-digital converter and, more particularly, to an analog-to-digital converter for converting an analog signal into a digital value in multiple steps.
2. Description of the Related Art
In recent years, a variety of additional functions are built in mobile appliances such as a mobile telephone set, including image pick-up function, image playback function, moving image pick-up function and moving image playback function. In association with this, there is an increasing demand for miniaturization and power saving of an analog-digital converter (hereinafter, referred to as an AD converter). One mode of AD converter that addresses this demand is known as a cyclic AD converter that cycles through stages (see, for example, patent document No. 1). Patent document No. 1 discloses an AD converter of a pipeline type comprising two blocks that include a conversion block of a cyclic type.
[patent document No. 1]
JP 4-26229
In the AD converter shown in FIG. 1 of patent document No. 1, a first switch SW1 is provided between a sample and hold circuit S/H2 of a first AD/DA block and a sample and hold circuit S/H3 of a second AD/DA block, and between the sample and hold circuit S/H2 and a parallel A/D converter AD2.
We have identified a problem associated with an AD converter of this type. Namely, if the sample and hold circuit S/H 3, for example, is embodied by a circuit with a capacitively coupled input such as a switched capacitor operational amplifier, the output of the operational amplifier used in the sample and hold circuit S/H2 is adversely affected by charges stored in a capacitor connected to the input terminal of the operational amplifier, causing settling time to be extended.